In civilian as well as in Military roles, it is often necessary to use an automatic radio direction finder (ADF) to determine the relative angle (relative bearing) with respect to the platform carrying the ADF to a source of radio frequency (RF) signal transmissions. For example, a tank, ship or aircraft can use an ADF to locate the source of enemy or friendly transmissions. The information provided by the ADF can thereafter be used for tracking and homing to the source of the signals. If accurate bearing information can be measured, the location of the source of RF signal transmissions can be estimated by making several bearing cuts or estimates of the source. A typical civilian and military usage is the determination of the relative bearing of a ship or aircraft to a remote radio transmitter. This may be used for example, to allow an aircraft to fly directly over the remote radio transmitter for navigational purposes.
Direction finders are typically provided with either a mechanically rotating antennae or an electronically scanned antennae. In the case of a mechanically rotating antennae, the receiving element is continuously rotated at a constant angular speed through 360 degrees. A directionally sensitive cardioid pattern is produced by the antennae such that when the antennae is facing broad side to an emitter, a distinctive "null" or point of minimum sensitivity is detected. The measurement of angular bearing to the emitter is then developed by the antenna's electronic circuitry.
Such mechanical systems require relatively large amounts of power, and have all the mechanical problems associated with physically rotating an antennae. Systems which derive bearing information by use of the null provide accurate bearing information when adequate signal strength is available and when the installed antenna has a normal ground plane with no reflections and no discontinuities. However, the primary disadvantage of the null type ADF used with a rotating antennae is that the signal frequently fades in and out and the antenna responds to variable signal reflections, thereby significantly reducing the usefulness of the intelligence. Another problem with conventional automatic direction finding systems which utilize the null of the antennae pattern is that under weak signal conditions, the signal strength of the received signals frequently approaches the sensitivity threshold of receiver, thereby broadening the width of the null and limiting the system accuracy. Furthermore, the time required to obtain a bearing is on the order of several seconds, creating a particular problem with uncooperative transmitters because the signal may not be present long enough to develop accurate bearing information.
To overcome some of the problems associated with mechanically rotating antennas, electronically scanned antennas have been developed. One such type of electronically scanned antennae is a multi-beam array antennae which includes an array of antennae elements, coupled to an electrical lens through constrained electrical paths. During operation with an ADF system, the typical principle of operation of an electronically scanned antennae differs from the rotating antennae in that the electronically scanned antennae seeks or senses a point of maximum received RF energy instead of the minimum or null seeking approach. In a typical electronically scanned antennae there are as many as eight separate antennae sections or sectors, each covering an azimuth value of 360/8 degrees. Electronic timing and logic rapidly selects each segment in a fixed sequence to achieve a step by step sweep of 360 degrees. An example of an ADF system having an electronically scanned antenna is the DF-301F UHF/VHF radio direction finder manufactured by Rockwell-Collins. This system is capable of operating in the frequency range of 100 to 400 MHz. Timing and logic circuitry is used to provide a desired antenna rotation frequency of 5.68 kHz. This sweep or rotation process occurs continuously in both a clock-wise and counter clock-wise rotation. When a sector or sectors of the scanned antennae detect maximum RF energy, electronic circuitry converts the received RF data into a relative bearing value.
Electronically controlled direction finders of the type described hereinabove perform accurately in theory, but often perform erratically and even inaccurately when installed on a vehicle such as an aircraft or ship. Noise and/or jitter are frequently observed, and angular errors as large as .+-.120 degrees are seen at various RF wavelengths. This erratic performance is extensively due to appendages and projections on a typical aircraft or ship that can attenuate, redirect and/or reflect RF energy. While the true RF signal is relatively constant, the attenuated or reflected signal is typically erratic or time variant. ADF systems typically scan 360 degrees at all times, and therefore receive and process the true RF direct path signal as well as the reflected or redirected components. The combinational processing of these signals produces noise, jitter, and other errors in determining the relative bearing of the transmitter to the ADF.